Field of the Invention
The invention relates to a direct sequence spread spectrum communication system, and more particularly to a code division multiple access receiver.
Description of the Related Art
Various wireless communication systems have adopted various schemes for supporting as many simultaneous users as possible. Code Division Multiple Access (CDMA) is one of such schemes. CDMA is a technique employed in spread spectrum communication systems that allows multiple users to simultaneously share the same frequency. In CDMA systems, a wideband spreading signal is used to convert a narrowband data signal into a wideband signal for transmission. Direct sequence spread spectrum (DSSS) systems use a pseudo noise (PN) sequence to spread the data signal into a wideband signal.
The signal to interference ratio (SIR) estimation is an important technique for CDMA receivers. In a CDMA receiver, many modern components utilize the SIR estimated result as a comparison threshold factor or as a decoding parameter, such as Multi-Path Searcher, Cell Searcher, Turbo Decoder, or Power controller. Especially in a power controller, the SIR estimation error is closely related to the power control error and therefore directly related to the service capacity. More precise SIR estimation improves system performance, and relies on more precise interference estimation.
Interference estimation estimates the interference power existing in received signals. The interference may include additive white Gaussian noise (AWGN) existing in passing channel, interference from other co-channel signals, interference from multi-path effects, interference from non-ideal demodulation process, and/or any other form of interference. One interference estimation method firstly re-builds the pure-signal component of the received signal in the receiver, and then subtracts the re-built pure-signal component from the total received signal to obtain an estimated interference component power. This method, however, is highly dependent on channel estimation precision to rebuild a reliable pure-signal component. Precise channel estimation is difficult in fast fading channels because the channel estimation algorithm may not be reliably for tracking the rapidly varying channel effect.
Another method utilizes the characteristics of orthogonal spreading code in the CDMA system. Because CDMA systems use orthogonal codes to distinguish different channels or users, and there always exists un-used orthogonal code, this method uses one un-used code to de-spread received signal for canceling the pure-signal component. For example, assuming interference is estimated by common pilot channel, and transmit diversity exists. In WCDMA systems with transmit diversity mode, common pilot symbols are orthogonally transmitted on 2 antennas as shown in FIG. 1. Another pattern, orthogonal to pilot symbol patterns on both antennas, can cancel the pure-signal by de-patterning the received symbol as shown in FIG. 2. In FIG. 2, pattern 21 is the pattern transmitted by antenna 1, pattern 22 is the pattern transmitted by antenna 2, and pattern 23 is the pattern for de-patterning the received symbol. Assuming that pattern 21 and pattern 22 are transmitted in a static channel, Hant1,0˜Hant1,9 are the channel response of each symbol of slot 0 in pattern 21, and Hant2,0˜Hant2,9 are the channel response of each symbol of slot 0 in pattern 22, then Hant1,0=Hant1,1= . . . =Hant1,9 and Hant2,0=Hant2,1= . . . =Hant2,9. The averaged de-patterning output for antenna 1 is 0, and the averaged de-patterning output for antenna 2 is A2·Hant2,0/5. Because no channel estimation result is required, the described method is generally more robust in fast-fading channels. Although the method performs better in a high speed environment, large interference power estimation offset still occurs when the Doppler frequency is high as shown in FIG. 3. In FIG. 3, assuming that pattern 21 and pattern 22 are transmitted in a fast-fading channel, Δant1, Hant1,0˜Hant1,9 are the channel shift and channel response of each symbol caused by fading in antenna 1 and Δant2, Hant2,0˜Hant2,9 are the channel shift and channel response of each symbol caused by fading in antenna 2. The averaged de-patterning output for antenna 1 is A2·Δant1/2, and the averaged de-patterning output for antenna 2 is A2·(2Hant2,0+9Δant2)/10.
Methods and apparatuses capable of more accurate interference estimation in high speed environments are thus desirable.